The present invention generally relates to rotating analyzer type ellipsometers, and more particularly to an ellipsometer which detects a phase difference between a rotating analyzer and a rotary phase detecting apparatus (rotary encoder) and sets an initial value for measurement calculations of an optical constant, film thickness and the like.
Generally, an ellipsometer which measures the ellipticity of the polarized light by use of ellipsometry, has been used conventionally when measuring the film thickness of a sample, for example. The ellipsometer is used for such a measurement because the sample will not be destroyed and it is possible to measure with a high accuracy the optical constant of the sample and the film thickness of an extremely thin single layer film by observing the change in the polarization state of the reflected light from the sample. An ellipsometer which employs a rotating analyzer is often used as an ellipsometer for performing such a measurement.
A general film thickness measuring apparatus which uses the polarization analysis method as the operating principle thereof, impinges light from a light source on a measuring plate with an arbitrarily selected incident angle. The measuring plate is formed with a transparent film on top of a transparent substrate, and the thickness of the transparent film is to be measured. The light reflected from the measuring plate is detected in an ellipsometer of the film thickness measuring apparatus, and a detection output of the ellipsometer is analyzed in an analyzer system so as to measure the film thickness of the transparent film.
In this type of a film thickness measuring apparatus, the film thickness is not measured immediately. First, an amplitude ratio tan .psi. and a phase difference .DELTA. between two mutually perpendicular polarized light components of the reflected light which is obtained when the incident light is reflected by the measuring plate, are compared. On the other hand, different values for the film thickness are successively substituted into a predetermined equation which has a film thickness d as the parameter, so as to obtain the amplitude ratio and the phase difference between the two polarized light components by calculation. It is assumed that the value for the film thickness d which is substituted into the predetermined equation is the thickness of the transparent film which is measured, when the calculated amplitude ratio and the calculated phase difference are equal to the measured amplitude ratio and the measured phase difference, respectively (with an error within a tolerance).
In the ellipsometer, the light impinged on the sample surface with a predetermined incident angle and reflected thereby is impinged on the rotating analyzer which is provided coaxially to the rotary encoder, and the light from the rotating analyzer is received by a photodetector. The incident angle with which the light is impinged on the sample surface must accurately coincide with the predetermined incident angle. Further, the accurate phase difference between the rotating analyzer and the rotary encoder must be known. These conditions must be satisfied because it is necessary to first set an initial value for the measurement calculation by use of the phase difference between the rotating analyzer and the rotary encoder when starting the measurement of the optical constant, film thickness and the like.
However, in an optical system in which the incident angle is fixed, there is no known method of measuring the phase difference between the rotating analyzer and the rotary encoder provided in the optical system. Hence, in the conventional ellipsometer, the phase difference between the rotating analyzer and the rotary encoder is detected by impinging the light from the light source directly on the rotating analyzer without by way of a reflecting surface. The light is a linearly polarized light of which polarization state is known.
But even in the case of the ellipsometer in which the phase difference between the rotating analyzer and the rotary encoder is detected by impinging directly on the rotating analyzer without by way of the reflecting surface the light from a laser light source which emits a linearly polarized light the polarization state of which is known, the light from the laser light source must be impinged on the sample surface with an incident angle accurately coinciding with the predetermined incident angle and the reflected light from the sample surface must be correctly impinged on the rotating analyzer through a pinhole, when the detected phase difference is to be used for the actual measurement. Accordingly, the mounting angles of a part including the light source and a part including the rotating analyzer must be set to respective desired mounting angles with an extremely high accuracy, from a state where the phase difference between the rotating analyzer and the rotary encoder is detected by impinging on the rotating analyzer the light from the light source without by way of the reflecting surface and with optical axes of the two parts coinciding, to a state where the light from the light source is impinged on the sample surface with the predetermined incident angle. But there are disadvantages in that such setting and adjustment are troublesome and difficult to perform. Moreover, mechanisms for permitting the mounting angles of the two parts to be variably adjusted with such a high accuracy, become complex and must be precise. As a result, the degree of freedom with which the designing may be carried out becomes poor, and the manufacturing cost becomes high. In addition, the mounting angles may become out of order and deteriorate the measuring accuracy. Further, there is a disadvantage in that a difficult operation of matching the optical axes of the two parts must be carried out when replacing a worn-out part.
On the other hand, according to the conventional film thickness measuring apparatus, when the amplitude ratio and the phase difference of the two polarized light components are taken along the X and Y coordinates and the film thickness is obtained with respect to the amplitude ratio and the phase difference is plotted, the collection of the plots form an oval shape. Thus, two values for the film thickness exist with respect to one phase difference, for example. For this reason, the film thickness cannot be obtained solely from the phase difference, and the film thickness must always be obtained from the amplitude ratio and the phase difference. Further, even when the amplitude ratio and the phase difference change slightly in value, the value of the film thickness which is obtained changes greatly. As a result, there are disadvantages in that the film thickness measuring accuracy is poor, and that it takes a considerably long time to perform calculations and the like for obtaining the film thickness.
Accordingly, the prevent inventors have previously proposed in a U.S. patent application Ser. No. 736,938 entitled "Film Thickness Measuring Apparatus" a film thickness measuring apparatus which impinges light from a light source on a transparent film with an incident angle .theta. equal to or approximately equal to a polarizing angle .theta. (.theta.=tan.sup.-1 n) which is determined by the refractive index n of the transparent film and measures light reflected by the transparent film. As will be described later on in the present specification, the present invention is suited for application to this previously proposed film thickness measuring apparatus.
Further, in a conventional rotating analyzer type ellipsometer, the axes of rotation of the rotating analyzer and the rotary encoder are independent of each other. In this conventional rotating analyzer type ellipsometer, the rotational force is transmitted from a rotary shaft of a motor to rotary shafts of the rotating analyzer and the rotary encoder by use of gears or timing belts, for example, since the axes of rotation of the rotating analyzer and the rotary encoder are independent of each other. In the case where the rotational force is transmitted by use of gears, an error is introduced between the rotary angle of the rotating analyzer and the rotary angle of the rotary encoder due to eccentricity and backlash of the gears. On the other hand, in the case where the rotational force is transmitted by use of timing belts, an error is introduced between the rotary angle of the rotating analyzer and the rotary angle of the rotary encoder due to the expansion and contraction of the timing belts. For these reasons, it is conventionally impossible to obtain a highly accurate measured result, and there was a demand for eliminating this problem.